1) Field of the Invention
This invention relates generally to semiconductor fabrication and more particularly to a method and chemical solution for preparing SEM samples comprising low-K dielectric materials.
2) Description of the Prior Art
For advanced generation semiconductors (0.18 microns and beyond), RC delay (resistancexc3x97capacitiance) begins to dominate overall device delay. Copper lines and low-K dielectric materials are attractive for reducing RC delay, and thereby enhancing performance. Chemical vapor deposited fluorinated oxide (FSG), hydrogen silsesquioxane (HSG), and methylsilsesquioxane (MSQ) spin-on materials are particularly attractive low-K dielectric materials for sub-quarter micron applications that are sensitive to RC delay, such as inter-level dielectric (ILD) layers.
However, due to different material characteristics, the etch rate for spin-on low-K materials is typically faster than the etch rate for traditional silicon oxide (SiOx), making it difficult to prepare SEM samples.
The importance of overcoming the various deficiencies noted above is evidenced by the extensive technological development directed to the subject, as documented by the relevant patent and technical literature. The closest and apparently more relevant technical developments in the patent literature can be gleaned by considering the following patents.
U.S. Pat. No. 4,733,074 (Kato et al.) shows a SEM sample surface structure having an insulating layer over the substrate.
U.S. Pat. No. 5,726,454 (Chun) shows a tripod for preparing a SEM sample.
U.S. Pat. No. 5,352,898 (Mehta) shows a method and apparatus for preparing slurry specimens for SEM.
Also, further relevant technical developments can be gleaned by considering the following articles.
Peters, xe2x80x9cPursuing the Perfect Low-K Dielectric,xe2x80x9d Semiconductor International, September 1998, pp. 64-74, discusses low-K dielectrics, including HSG.
Albrecht, xe2x80x9cMaterial Issues with Thin Film Hydrogen Silsesquioxane Low-K Dielectrics, J. Electrochem. Soc. Vol. 145, No. 11, November 1998, pp. 4019-4024, discusses chemical composition of HSG and its effect on dielectric constant and thermal stability.
It is an object of the present invention to provide a method and chemical solution for preparing SEM samples comprising low-K spin-on dielectric materials.
It is another object of the present invention to provide a method and chemical solution for staining SEM samples which provides a decreased difference in etch rates of low-K spin-on dielectric materials and convention silicon oxides. It is yet another object of the present invention to provide a method for preparing a chemical solution for staining SEM samples which provides a decreased difference in etch rates of low-K spin-on dielectric materials and convention silicon oxides.
To accomplish the above objectives, the present invention provides a method and a solution for preparing SEM samples comprising low-K dielectric materials. The process begins by providing a SEM sample comprising low-K dielectric material and silicon oxide material. A solution is formed for preparing (staining and etching) the SEM sample by adding NH4F (s) to a solution comprising CH3COOH having a concentration of about 90% at a ratio of about 1 g NH4F (s):20 ml CH3COOH, then stirring until the NH4F (s) is thoroughly dissolved. Alternatively, the NH4F (s) can be added to a solution comprising HNO3 having a concentration of about 70% and CH3COOH having a concentration of about 90%, with a volume ratio of about 15 ml HNO3:20 ml CH3COOH. The NH4F (s) is added at a ratio of about 1 g NH4F (s):35 ml CH3COOH and HNO3, and stirred until the NH4F (s) is thoroughly dissolved. The SEM sample is then etched in this solution for about 3 seconds.
The present invention provides considerable improvement over the prior art. Most importantly, SEM samples comprising low-K dielectric materials and silicon oxides can be prepared wherein the low-K dielectric material and silicon oxide material have similar etch rates with good selectivity to metals.
The present invention achieves these benefits in the context of known process technology. However, a further understanding of the nature and advantages of the present invention may be realized by reference to the latter portions of the specification and attached drawings.